Dungeon Difficulty

I'd be interested to know - what determines whether a dungeon is shown easy, middle or hard (with the symbol in the top left corner). I've just entered the tumulus and my symbol is green, but even the monsters at the start are really hard. I do have quite good equipment (although not the best for all as I never seem to get enough money), yet the monsters are dealing a lot of damage and don't go down easily. They also seem to be quite immune to states.